Those who have not played "first-person shooter" games like this may not understand how realistic they have become in recent years. Because this topic has now become part of the national debate on how to reduce or stop gun massacres, we will walk through an example of these games.

Importantly, we have no idea what short-circuited in Lanza's head that made him decide to use real guns to kill real people in Newtown last week. But as you will see, the experience of walking around shooting people would not have been unfamiliar to him.

The first point to keep in mind is that Lanza is not the only one who played "Call of Duty." The best-selling video game series has around 40 million monthly users, including myself — and personally I doubt that it makes people turn violent.

Nonetheless, "Call of Duty" is widely considered one of the most realistic combat games out there.

As a Marine, I can confirm that, while nothing comes close to actual combat, the game offers a glimpse into the world of armed killing. It gives a realistic portrayal of what happens when someone gets shot. It also conveys the speed of combat and how instant reflexes can mean life or death. Indeed this is why games like "Call of Duty" are used by the military to train soldiers.

Though there is a single-player or two-player campaign mode, most fans buy the game to play in the online arenas against other human players. In the following slides, we will walk you through a"Call of Duty" "kill streak" posted to YouTube by a player with the handle stojvip3r11.

One word of caution: These images are graphic, and in light of what happened last week, they may seem distasteful. But they are the reality of a highly popular genre of video games, which is why we are presenting them here.